Abstract Multiple myeloma (MM) is an incurable hematological malignancy characterized by accumulation of abnormal plasma cells in the bone marrow (BM) which impede production of normal blood cells. The average survival of MM patients has improved in recent years as a result of introduction of proteasome inhibitors and immunomodulatory agents into treatment regimens but is still quite poor at only 5 years. FDA approval of the proteasome inhibitor bortezomib validates the ubiquitin proteasome system (UPS) as a therapeutic target for MM drug development. Recent research efforts have focused on the discovery and development of small molecule inhibitors of other major components of the ubiquitin proteasome system including deubiquitylating (DUB) enzymes which remove degradative ubiquitin signals from protein substrates. The DUB ubiquitin specific protease 7 (USP7) has garnered interest as a therapeutic target in MM due to its role in degradation of tumor suppressor protein p53. USP7 stabilizes MDM2, the protein responsible for adding ubiquitin and driving degradation of p53. Importantly, USP7 is highly expressed in MM patient tumor cells and MM cell lines versus normal primary BM cells. And mutations or deletions in p53 are late events in MM suggesting that increasing p53 via pharmacological inhibition of USP7 may offer a novel therapeutic strategy for this malignancy. We recently reported on the effects of the covalent USP7 inhibitor P5091 on p53 stabilization and growth suppressive effects against MM cells ex vivo and in MM xenograft models. Moreover, P5091 triggered apoptosis even in bortezomib-resistant MM cells. Overall, our results validate USP7 as a therapeutic target in MM. To further assess the potential of USP7 as a target in MM and move USP7 inhibitors from bench to bedside, we propose to utilize USP7 structure-guided medicinal chemistry to develop more potent and selective USP7 inhibitors with improved pharmacokinetic properties and in vitro safety profile. Novel USP7 inhibitors will be evaluated for efficacy using both in vitro and in vivo models of MM and in primary human samples. A multi-disciplinary team has been assembled to perform the medicinal chemistry (Sara Buhrlage, Dana-Farber Cancer Institute), biochemistry and structural biology (Sirano Dhe-Paganon, Dana-Farber Cancer Institute), and cell and cancer biology in MM models (Kenneth Anderson and Dharminder Chauhan, Dana- Farber Cancer Institute) required to pharmacologically interrogate USP7 in MM.